Resistor substrate having integral metal terminations



1967 E. w. STAPLETON ETAL 3,

RESISTOR SUBSTRATE HAVING INTEGRAL METAL TERMINATIONS Filed April 29,1965 FIG. 4

28 FIG 2 2/ 24 I I 2m 23 23 FIG. 3 /2- /3 /4 CE 221, C WET ORV/N6GRIND/N6 MATERML BALL A/vo A/v0 BATCH/N6 M/X/NG CALC/N/NG M/Xl/VG /7CERAMIC AND J5 METAL POWDER M/X/A/G 5pm y I SCREENING DR y/NGM/l/EA/TORS PRESS/N6 IRVIN J. McKEAND FIR/N6 70 FORM EARL W. STAPLETONSUBSTRATE By WM AGENT United States Patent Ofilice 3,360,761 RESISTORSUBSTRATE HAVING INTEGRAL METAL TERMINATIONS Earl W. Staplcton,Syracuse, and Irvin J. McKeand,

Fayetteville, N.Y., assignors to Air Reduction Company Incorporated, NewYork, N.Y., a corporation of New York Filed Apr. 29, 1965, Ser. No.451,917

4 Claims. (Cl. 338-330) ABSTRACT OF THE DISCLOSURE A mechanicallydurable and electrically stable electrical resistor is formed by coatinga fully vitrified ceramic substrate having integrally formed metallicterminals with a resistive composition of glass in which inorganicconductive particles are dispersed. The resistive composition is firedin situ on the prefired substrate to form a completely vitrifiedresistor unaifected by temperature, moisture or surrounding atmosphere.

The present invention relates to electrical resistors and the method ofmaking the same. More particularly, the invention relates to resistorswhich comprise a substrate having an electrically non-conductive ceramicportion and integral conductive metal or metal-ceramic mixture (cermet)terminal portions, and a resistive coating deposited on thenon-conductive portion of the substrate and electrically connecting theterminal portions. Wire leads are preferably attached to the conductiveterminal portions.

Prior art film resistors generally consist of a solid ceramic substrate,a resistive coating applied thereto, and metal end-caps slidably mountedover the end portions of the resistive coating with the lead wiressoldered or otherwise suitably aflixed to the end-caps. At this point,it should be noted that any reference herein to a film resistor isintended to include resistors which consist of a substrate which iscoated with metal, metal oxide, or carbon thin films, or with a thickercoating such as an inorganic composition comprising a glass and metal ormetal oxide mixture. The coating may completely enclose the substrate orit may be deposited on the substrate in a spiral or other desiredconfiguration. United States Patent No. 1,814,583 illustrates a typicalprior art resistor with metal end-caps. The necessity of such end-capspresents many disadvantages in both the use and manufacture of filmresistors. The use of end-caps automatically creates an additional stepin the assembly of a resistor. The endcaps are generally of thin,delicate construction, particularly when used with resistors of smallphysical size, and presents serious difiiculties in attaching a wirelead thereto. In the use of the resistor, the thin flanges are easilydamaged if the leads are accidentally pulled during circuitinstallation, and the leads may often completely detach from the endcaps. Further, in modern compact electronic equipment where size is ofextreme importance, the overlapping flange of the end-caps results in alarge and cumbersome resistor which is undesirable. Additionally, endcaps are expensive and substantially increase the cost of the resistor.

It is an object of the present invention to provide improved filmresistors for use in electronic circuits. It is another object of thepresent invention to provide film resistors which are extremelyreliable. It is another object of the invention to provide filmresistors which eliminate the necessity of metal end-caps. It is stillanother object of the invention to provide film resistors which aresmall in size and which may be manufactured economically. It is afurther object of the invention to provide a resistor comprising asubstrate having a ceramic portion and integral metal or cermet terminalportions. It is a further object of the invention to provide a resistorcomprising a substrate having a ceramic portion and integral metal orcermet terminal portions fired in situ with the center portion. It isyet another object of the invention to provide a film resistorcomprising a substrate having a ceramic portion and integral metal orcermet terminal portions in which wire leads are securely imbedded. Itis a further object of the invention to provide a film resistorcomprising a substrate having a ceramic portion with integral metal orcermet terminal portions and wire leads welded, brazed, or otherwisesecurely fastened to the terminal portions. It is an additional objectof the invention to provide a film resistor comprising a substratehaving a ceramic portion with integral metal or cermet terminal portionsfired in situ therewith, with the ends of the conductive filmoverlapping and electrically connecting the terminal portions. It isanother object of the invention to provide a method for making the newand improved resistors of the present invention. Other objects,features, and advantages of this invention will be readily apparent fromconsideration of the following specification relating to the annexeddrawings in which:

FIG. 1 is a sectional longitudinal view of one embodiment of theinvention;

FIG. 2 is a partly elevational and partly sectional longitudinal view ofa second embodiment of the invention;

FIG. 3 is a flow diagram showing the general steps by which the resistorof our invention is manufactured;

FIG. 4 represents a vertical section through a mold showing the methodof assembling the powdered end por tions and the powdered center portionof the substrate prior to the substrate being formed in the moldingoperation; and

FIG. 5 represents a vertical section through the mold similar to FIG. 4,but showing the assembled materials after they have been pressed.

Referring to the drawings, FIG. 1 is a view in section of one form ofthe resistance unit. The body of the resistor is indicated at 1 and ispreferably cylindrical in form as shown, although other configurationsare possible and within the scope of this invention. The body of theresistor is formed by a substrate having an electrically nonconductivecenter portion 2 and conductive metal or cermet terminal end portions 3,the substrate having a substantially uniform diameter along its length.It is obvious, however, that the substrate may take other forms withoutdeparting from the scope of this invention. A conductive material 4 isapplied to the center portion of the substrate and overlaps the endportions 3 as shown at 8. Wire leads 5 are embedded in the terminations3 as shown at 6. Finally, the resistor is coated with a suitableprotective coating 7.

The resistor shown in FIG. 2 is identical to the resistor of FIG. 1 withthe exception of the manner in which the wire leads 5 are electricallyconnected to the end portions 3. Consequently, like parts of FIGS. 1 and2 are designated by the same reference numerals. In FIG. 2, leads 5 arewelded, brazed, soldered, or the like, to end portions 3 as shown at 9.

In manufacturing the resistor, with reference to FIG. 3, raw materialsnecessary for the preparation of a ceramic formulation are weighed andbrought together in a raw material batching operation as at 11. Thematerials are then mixed in some convenient manner such as by wet ballmilling indicated at 12. An example of a satisfactory formulation forthe ceramic is 78% by weight prefused forsterite, 5% barium carbonate,5% calcium carbonate, and 12% silica. Other ceramic formulations mayobviously be advantageously employed if selected so as to be compatiblewith the terminal portions and the resistive film. The mixed ceramic isthen dried and calcined at 13 by heating to some temperature at whichvitrification is imminent, i.e. in the range of from 2100 F. to 2400" F.The calcined ceramic is ground at 14 by any well known method such as ina dry ball mill and prepared for pressing by some standard technique.The ground material may be mixed with an organic binder in the ball millbefore pressing if desired, but it is not necessary to the procedure. Inthe event a binder is used, a satisfactory composition is a mixture ofwater, 0.2% calcium stearate, and 1% methocel, which is mixed with thecalcined ceramic in the ball mill. At the conclusion of mixing, theceramic is spray dried as at 15 to from to 1% moisture by weight. Themix is then screened at 16 to produce the proper powder consistency. Toprovide cermet terminal portions, a portion of this ceramic is mixed at17 with a metal powder such as steel, nickel, iron, etc., in a suitableproportion to provide both the desired electrical conductivity and thedesired physical properties. The metal used in the mixture is determinedby the firing temperature of the ceramic and the proportions aredependent upon mixing efficiency and relative particle size of theceramic and metal. In the preferred mixture, the metal and ceramic arein the range of 2 to 50 microns in size. The cermet mixture and pureceramic mixture are then brought together and pressed at 18 to form theunitary structure as shown in FIG. 1, or as shown in FIG. 2, dependingupon the manner in which the lead wires are to be attached to the cermetends. The pressing operation is fully disclosed below. If pure metal ormetal alloy terminal portions are to be used, then the ceramic and metalpowder mixing at step 17 in FIG. 3 is obviously unnecessary. Metalpowder alone is used in the pressing operation 18 for the terminalportions.

As previously noted, pure metal or metal alloys may be used for theterminal portions. Whether cermet or metal end portions are used, thecriteria in selecting the particular terminal portion is the same inthat terminal portion material must be matched to certain physicalcharacteristics of the ceramic center portion so that a suificientlystrong bond between the center portion and terminal portions may beobtained. The terminal portion material must also be highly conductive(since it forms electrical terminals) and it should be capable offorming a good weld joint when lead Wires are welded thereto. I havefound that, in the case of the cermet material, the amount of metal mustexceed 50%, and preferably exceeds 65%, of the total weight of thecermet mix in order to obtain the necessary conductive and weldingcharacteristics. Nickel, iron, and steel have been found satisfactoryfor use in the cermet terminal material. In the case of metal terminalportions, an alloy consisting of 45% to 55% by weight iron, 55% to 45%by weight nickel, and up to by weight molybdenum, has been found toproduce satisfactory bonding strengths with ceramic when processedaccording to the procedures set forth in FIG. 3 of the drawing.

Following the pressing operation, the unit is placed upon or in asupporting refractory as at 19 and fired t0 the vitrification point in areducing atmosphere. For example, the unit is set in a V-groove vat andfired in disassociated ammonia in the temperature range of 2300 F. to2600" F. The firing produces a rod having a vitrified ceramic centerportion with end portions of sintered metal-ceramic when a cermetmixture is used. The metal particles in the cermet end portions providea continuous path for thermal and electrical conductivity and theceramic present being vitrified holds the metal together, bonds the discto the ceramic rod, and provides compatible coelficients of thermalexpansion to the ceramic rod. When metal alloy end portions are formed,it is found that the metal is sintered to the ceramic center portion insuch a manner that extremely high bonding strength is obtained. The unitis then cleaned and a metal flash such as gold or silver is placed overthe metal ceramic terminal portions, if desired, by plating, painting,or other convenient techniques to prevent oxidation of the terminalportion during application of the resistive coating to the substrate. Ifthe lead wires were not previously inserted in the end portions, theyare now welded, brazed, soldered, or inserted into or against the endportions.

If the film resistor of FIGS. 1 and 2 is to be made, a layer ofresistive material is now paint sprayed, vacuum deposited, or otherwiseplaced over the unit and given subsequent treatment as necessary to meetthe resistor requirements for production and electrical properties. Apreferred example of a resistive material which has been found to beparticularly adapted to the disclosed substrate is fully disclosed inthe pending application of Kee Hyong Kim, Ser. No. 286,202, filed June7, 1963, entitled Resistor Manufacture, and assigned to the assignee ofthe present application. The preferred resistive coating disclosed inthe above-identified application is an inorganic composition containingthallium oxide as the major conductive component dispersed in a glassymatrix. The preferred proportions of thallium oxide to matrix arethallium oxide 95% to 20% by weight and glassy matrix 5% to by weight.Other resistive coatings that may be used on the disclosed substrate aremixtures of glass and any one or any combination of the metal oxides,such as ruthenium oxide, silver oxide, tantalum oxide, titanium oxide,palladium oxide, and the above-mentioned thallium oxide. Any one of thecompositions disclosed in United States Patent No. 3,052,573 may also beused as the resistive coating. It is obvious that many other resistivecompositions will be equally useful as a coating for the disclosedsubstrate. If desired, the resistance of the component may be increasedby spiraling or otherwise removing selected areas of the conductivecoating to thereby increase the length of the electrical path betweenthe conductive terminals.

The resistor is made into a finished product by applica tion of aplastic or other non-conductive jacket by any well-known moldingprocess, such as by hot molding of epoxy resins, alkyd resin, or diallylphthal'ate resin. In addition, epoxy resin or epoxy-silicon resin may beapplied by conformal coating techniques.

The pressing apparatus necessary to carry out the pressing operation 18of FIG. 3 and to produce the resistor of FIGS. 1 and 2 is illustrated inFIGS. 4 and 5. The apparatus consists of a die block 21 having avertical die opening 14 therethrough in which the resistor will bemolded. In the drawings, a single die opening is shown but in commercialpractice the die block properly would be provided with a large number ofsymmetrically arranged die openings. The bottom plate 23 is providedwith a stem 24 which fits into the die opening 22 with a close slidingfit. The apparatus also includes a top plate 21 and a pin 20 which alsofits into the die opening 22 with a close sliding fit. In the moldingoperation with the stem 16 inserted part way into the die opening 14 asshown in FIG. 4, a measured quantity of metal or cermet powder 25 ispoured into the die opening. On top of this powder is poured the correctamount of ceramic 26 and on top of the ceramic is placed an additionalquantity of the metal or cermet powder 19. In accordance withconventional practice, the die block will be rapped or vibratedfollowing the pouring in of each of these materials to insure settlementin uniform layers.

Next the pin 28 projecting from the top plate 29 is inserted in the dieopening and hydraulic pressure is applied to the plates 23 to 29 by apress in a known manner. The pressure employed will be sutficient toprovide a high density in relation to the fired density with thepreferred range of pressure being 5 to 10 tons per square inch.Following the molding operation, the resistor will be ejected from themold, for example, by the stem 16 which is long enough to push themolded resistor out of the die block. The relative position of the partsof the pressing apparatus are illustrated in FIG. 5, following themoment of press and just prior to the ejection of the resistorsubstrate.

If lead wires are to be embedded in the cermet portions of the substratethe wires may be inserted at the moment of press by any well-knownmethod. For example, the pins of FIGS. 4 and 5 may be hollow to hold thelead wires in a manner as fully explained in United States Patent No.3,013,240.

It will be understood that the invention herein disclosed may bevariously mod'fied and embodied within the scope of the followingclaims.

We claim:

1. A fully vitrified electrical resistor comprising a substrate having anon-conductive ceramic center portion and conductive end portions formedfrom powders pressed and fired together into a vitrified unitarystructure, and a resistive composition supported thereby to form anelectrically continuous path from one conductive end portion of thesubstrate to the other, the said resistive composition comprising glassfired to vitrification after application to the prefired substrate andhaving finely divided conductive particles dispersed therein.

2. An electrical resistor according to claim 1 in which wire leads arewelded to the metal containing conductive end portions of the substrate.

3. An electrical resistor according to claim 1 in which the finelydivided conductive particles dispersed in the 25 glass are conductivemetal oxide particles.

conductive particles and firing the so coated unit to vitrify thecoating material and form an electrically conductive resistive coatingcomprising conductive particles in a glass matrix in an electricallycontinuous path from one conductive end portion of the said substrate tothe other and to simultaneously bond the said coating to the saidsubstrate.

References Cited UNITED STATES PATENTS 3,192,497 6/1965 Bender et al338-331 3,220,097 11/1965 'Griest 338237 X 3,238,151 3/1966 Kim 2525l83,295,090 12/1966 Hay 338270 X RICHARD M. WOOD, Primary Examiner. I. G.SMITH, Assistant Examiner.

1. A FULLY VITRIFIED ELECTRICAL RESISTOR COMPRISING A SUBSTRATE HAVING ANON-CONDUCTIVE CERAMIC CENTER PORTION AND CONDUCTIVE END PORTIONS FORMEDFROM POWDERS PRESSED AND FIRED TOGETHER INTO A VITRIFIED UNITARYSTRUCTURE, AND A RESISTIVE COMPOSITION SUPPORTED THEREBY TO FORM ANELECTRICALLY CONTINUOUS PATH FROM ONE CONDUCTIVE END PORTION OF THESUBSTRATE TO THE OTHER, THE SAID RESISTIVE COMPOSITION COMPRISING GLASSFIRED TO VITRIFICATION AFTER APPLICATION TO THE PREFIRED SUBSTRATE ANDHAVING FINELY DIVIDED CONDUCTIVE PARTICLES DISPERSED THEREIN.